Field of the Invention
Embodiments of the invention relate to processes for purification of water.
Background of the Related Art
Vertical tube falling evaporators are conventionally used for recovery of produced water generated in steam flood processes used for heavy oil extraction. The produced water is purified by distillation and used for boilers for generating steam. The steam is injected in the underground well to recover oil, which comes out mixed with produced water. Oil is separated from water. During oil extraction the produced water picks up significant amount of dissolved solids including hardness and silica, and dissolved color and organics. This is usually purified by distillation after de-aeration and then taken for steam production again. During the distillation a goal is usually for more than 95% water to be recovered. The balance is discharged in salts caverns or injected in deep wells.
This way overall consumption of water is optimized with minimum make up losses. This process of producing oil is called the “Steam Flood” or “Steam Assisted Gravity Drainage” Process (SAGD) or “Cyclic Steam Stimulation” (CSS). The water treatment is a critical part of this operation. The water treatment process requires careful design to prevent or at least minimize scaling and solid build up, to increase up time of operation, and to improve reliability.
In existing evaporative water treatment processes for steam flood, water is pretreated to remove oil and subsequently the pH is raised to precipitate part of the hardness and keep silica in dissolved form. The part of precipitated hardness remains in the settlement tank and there is no carry-over of sludge in the evaporation system. In such a process there are no solids in the evaporation mixture and the distribution system. This water is distilled to recover almost 95-97% water through vertical falling film evaporators, and the residual brine is neutralized to reduce the pH as per environmental regulations and discharged in salt caverns or through deep well injections.
In another evaporative water treatment process, after oil removal silica is precipitated with addition of magnesium oxide and sodium hydroxide at alkaline pH, where silica is adsorbed on the surface of magnesium hydroxide. In this process resultant precipitated magnesium and silica sludge does not leave the system when water is taken for evaporation. Sludge, including color and organics as a part of water, is continuously re-circulated through the evaporator through vertical tube falling film evaporators. This risks settlement of part of the sludge in the distribution system on a continuous basis. Over a period of time this may result in formation of deposits on the surface of tubes and scaling of the tubes.
After the distillation and recovery of 97% water the brine is taken for neutralization. During the neutralization, most of the silica exists in precipitated form, which may reduce chances of gel formation but requires that all of the solids be disposed of with brine, which make the process very expensive and consumes a lot of capacity of salt cavern. Brine generated through this process is not suitable for deep well injection without extensive treatment for solids removal at this stage, where solids have sluggish settling and filtration characteristics.
Unfortunately these processes have proven unsatisfactory for a number of reasons. These include scaling of evaporator surfaces and creation of a more chemically-intensive waste product for disposal to underground wells or other areas.